Liquid chromatography is a technique in which liquid samples to be analysed are injected into a stream of solvent flowing through a chromatographic column. The sample size to be injected is usually several microliters in volume, but it may vary in size from less than 1 microliter up to 100 microliters or more. The pressure of the solvent stream into which the samples are injected may vary from less than 1000 psi up to 6000 psi or more. It is frequently necessary to analyse large numbers of samples routinely. Automatic sample injection devices are used in these situations. In many cases, the samples are available only in small quantities because they are difficult to isolate.
Automatic sample injection systems are available which make use of a conventional 6-port sample injection valve with a sample loop which must be filled with the sample fluid. Such a device is shown in U.S. Pat. No. 3,918,913 to Stevenson & Coffey. These systems require an appreciable excess of sample to fill the sample loop reliably, and the sample liquid left behind in the connecting tubing between sample vial and valve must be discarded to make way for the next sample. Thus they may use more sample volume than is readily available.
Recent designs of manually operated sample injection devices make use of micro syringes to draw up a sample from a vial and introduce it into an injection valve with little or no loss of sample. Examples are U.S. Pat. Nos. 3,916,692 to Abraham and Hutchins, 3,961,534 to Gundelfinger, and 4,022,065 to Ramin and Stearns. These devices, however, are difficult to automate because the syringe has to be flushed several times between each injection in order to eliminate cross contamination.
There exists a need, therefore, for a sample injection system with the following features: (a) the sample volume can be varied over a range from less than 1 to 100 microliters, (b) there is very little or no sample wastage, and (c) automatic operation can be conducted with high precision and without cross-contamination.